The present invention concerns an apparatus for measuring the impedance of an unknown device by phase comparison of the current through the device with the voltage across the device.
In prior art methods for measuring the impedance of an unknown device, to measure the phase difference of the current through the device and the voltage across the device, a phase detector, or a synchronous rectifier, has been employed. But the phase detector has the disadvantage of having a constant phase offset error. This phase error is an unavoidable measurement error, because the error is generated in part by the phase difference between the two signal input paths to the phase detector. This error is also generated in part by a residual phase error caused by the phase detector itself.
Another disadvantage to the prior art impedance meter is the difficulty of obtaining more accurate measurement and better resolution in the measurement of dissipation factors D, or tan.delta. in the case of a capacitive impedance.
An apparatus for measuring the vector voltage ratio using a synchronous phase detector circuit and an analog circuit is shown in Japanese patent application number SHOWA No. 54-09358, entitled "An Apparatus for Measuring the Vector Voltage Ratio of Two A.C. Signals." The above cited application describes only the apparatus and method for measuring the vector ratio of two signals; it does not disclose a method for measuring impedance or dissipation factor D.
Accordingly, an object of the present invention is to provide an apparatus which can reduce measurement errors caused by phase detectors and which can increase accuracy and resolution in the measurement of the dissipation factor D of an unknown device.